Orthodontic appliances can generally be divided into two groups: "reactive" and "active". Traditionally, appliances such as brackets have been considered "reactive" appliances since their only purpose has been to physically hold in place a force generating "active" appliance such as an archwire. It is the recovery force of the archwire which results in tooth movement.
Orthodontic brackets have manufactured from materials such as stainless steel as disclosed in U.S. Pat. Nos. 4,536,154 and 4,659,309; ceramics as disclosed in U.S. Pat. Nos. 4,954,080, 5,011,403, and 5,071,344; certain types of plastics as disclosed in U.S. Pat. No. 4,536,154; or plastic composites as disclosed in U.S. Pat. No. 5,078,596.
Brackets made of ceramics or various types of stainless steels are generally rigid and strong. On the other hand, plastic materials, due to their relatively lower strength, exhibit permanent deformation during treatment. This failure is propagated by the stresses generated by the loading forces generated by "active" elements such as an archwire as well as by masticatory forces.
Brackets fabricated from polycarbonate demonstrate distortion under torsional loading generated by orthodontic archwires, and also possess a high propensity for water absorption, which may result in discoloration of the bracket and undesired staining (G. V. Newman, Am J. Orthod. 1969;56:573-588). These factors limit the use of such brackets in the oral environment.
Ceramic brackets are extremely brittle and even the smallest surface cracks (flaws) can dramatically reduce the load required for fracture (G. E. Scott, The Angle Orthodontist, 1988;58:5-8). Brackets that distort or fail during treatment render tooth movement ineffective and minimize control of tooth movement, thereby extending treatment time. Chemical retention of the ceramic bracket base to adhesive is generally facilitated by a coating of silica and silane coupling agent. The resultant chemical bond is very strong and may cause the enamel adhesive interface to be stressed during either debonding or sudden occlusal force. Hence, irreversible damage to the enamel of the entire tooth may occur and is particularly significant when bonding endodontically treated teeth or teeth with large restorations. (M. Schwartz, J. Clinical Orthod 1988;22:82-88). In addition, due to the hardness of ceramic brackets, abrasion during the chewing process can lead to enamel wear.
It has been suggested that adequate bond strengths for brackets should be in the vicinity of 5.9 to 7.8 MN/m.sup.2. With ceramics, bond strengths as high as 28.27 MN/m.sup.2 may be obtained which may compromise the safety margin of the stresses that can be withstood by the cohesive strength of enamel (V. P. Joseph et al., Am J Orthod, 1990;97:121-125). This may lead to enamel fracture. The incidence of fracture of ceramic brackets themselves is also of concern. It has also been reported to be as high as 6.66% (VPJ, ibid.). Pieces of bracket may be ingested or inhaled inadvertently if fracture occurs in the mouth during treatment.
On the other hand, it appears that stainless steel brackets begin to deform when shearing forces are applied. This leads to debonding of the bracket before reaching the cohesive fracture strength of the adhesive. This phenomenon prevents any enamel or bracket fracture. Steel failure, which can lead to bracket deformation or even breakage, has not been considered as dramatic.
While the mechanical strength of a bracket is an important design consideration, its corrosion resistance is equally important. This characteristic determines its biodegradation and the leaching of potentially harmful ions into the oral environment. Therefore the choice of a material that demonstrates high corrosion resistance while being biocompatible is vital for use in the oral environment.
In the process of bonding a bracket to them, teeth are conventionally treated by an acid-etch technique with subsequent placement of the orthodontic bracket. When using 304 stainless steel brackets, it was reported that the presence of voids together with poor oral hygiene led to crevice corrosion of 304 stainless steel and formation of colored corrosion products that resulted in enamel stains (R. Maijer et al., Am J. Orthod, 1982;81:43-48).
Both enamel staining, and the most serious problem of mucosal allergy, also due to the heavy metals leaked from corroded appliances, are phenomena which are often encountered and have been comprehensively studied. What makes the problem more dramatic is the fact that nickel, found in all the stainless steels used in orthodontics, produces more allergic reactions than all other metals combined; furthermore this ion is cytotoxic. Delayed hypersensitivity response to nickel containing stainless steel (e.g., AISI 304 and 316 stainless steel) has been reported (W. R. Schriver et al., Oral Surg, 1976;42: 578-581).
On the other hand, titanium and titanium based alloys are reported to have the greatest corrosion resistance of any known metallic materials. Implants in monkeys of commercially pure titanium (at least 99% titanium by weight), show no evidence of corrosion or release of Ti in adjacent tissues after being as much as 1 .sub.1/4 years (A. Schroeder et al., J Max-ac Surg 1981;9: 15-25). This is due to a more stable passive (oxide) films formed on Ti and Ti-based alloys. Related metals such as Cr and Co, and alloys of Cr and Co as well as alloys of Zr, Si, B, Be and Nb should offer advantages similar to that of Ti and its alloys.
Frictional resistance is another important design consideration of an orthodontic bracket. Translational tooth movement along an archwire requires sufficient force to overcome frictional forces between the bracket and archwire. Both the static and kinetic frictional forces generated between brackets and archwires during sliding mechanics should be minimized to allow optimal tooth movement during orthodontic mechano-therapy.
It is reported that the coefficient of kinetic friction for stainless steel (0.139) was less than that for the polycrystalline alumina bracket (0.174), with both measurements taken against stainless steel archwire (R. P. Kusy et al., Am J Orthod Dentofac Orthop, 1990;98:300-312). Although this ranking holds for both dry (air) and wet (artificial saliva solution) conditions, the coefficients of friction in wet environments generally show lower values than those in dry environments because the saliva serves itself as a lubricant (D. H. Pratten et al., Am J Orthod Dentofac Orthop, 1990;98: 398-403).
Moreover, when a stainless steel bracket was coupled with different types of archwire materials, the coefficients of kinetic friction ranged from stainless steel (lowest), to CoCr, TiNi, and .beta.-Ti (highest), regardless of bracket product or slot size and were 0.140, 0.163, 0.337 and 0.357, respectively (R. P. Kusy et al., Am J Orhtod Dentofac Orthop, 1990;98:300-312).
Therefore, it is desirable to provide an orthodontic bracket made from material(s) having excellent corrosion resistance and biocompatibility, low coefficient of friction, high value of strength to-weight ratio, and good bonding characteristics. It is also desirable to be able to simplify the design of orthodontic brackets by constructing them as a single piece.
An additional consideration is the role of orthodontic brackets when a patient is subjected to medical imaging techniques. Among adult patients wearing orthodontic brackets, 20-25% of the population may require surgery of some sort during the course of orthodontic treatment. Metals, particularly those that contain iron, are magnetic and are referred to as ferromagnetic materials. When brackets are comprised of such ferromagnetic materials, they interfere with MRI and CT imaging by creating scatter. Ti, particularly anodized Ti, is non-magnetic and thus would limit interference on the recorded image, thereby enhancing the reliability of such diagnostic images. Ti and its alloys, as well as Cr, Co and alloys of Cr, Co, Zr, Si, B, Be and Nb all should demonstrate these desired features. Ti performs advantageously in a range from 45 to over 99% (the latter being "commercially pure" titanium). Performance increases the higher the percentage of Ti in the alloy.